At least five Philippine soldiers have been killed and several injured in fresh clashes with Muslim militants, according to the security forces.
Troops are searching for several key militants
Several of the militants were also killed or wounded in the fighting, military officials said.
Troops have been fighting Abu Sayyaf rebels on the remote southern island of Jolo since late July in a bid to flush out leader Khadaffy Janjalani.
They are also searching for members of Islamic militant group Jemaah Islamiah.
Military officials said fighting began in the early morning in a mountainous area of the island.
Major General Eugenio Cedo of the Western Mindanao Command said troops believed that they had struck the main Abu Sayyaf group.
"We believe many of them were either killed or wounded because we've recovered clothes and bandoleers of ammunition stained with blood," Maj Gen Cedo told reporters.
Troops were still searching for the militants, who had fled further inland, he said.
Last week, the military sent a battalion of army rangers to bolster 3,000 troops already hunting the Abu Sayyaf rebels.
Abu Sayyaf is the smallest and most violent militant group in the Philippines. The US has listed it as a terrorist organisation and says it has links to al-Qaeda.
It is infamous for kidnapping Westerners and Filipinos, beheading victims and receiving large ransom payments. It was blamed for the bombing of a passenger ferry in 2004 which killed more than 100 people.
But troops are also hunting for two suspects in the 2002 Bali bombings, Dulmatin and Umar Patek, who are believed to be sheltering with Abu Sayyaf.
The two Indonesians belong to Jemaah Islamiah, a regional Islamic network with alleged links to al-Qaeda.
There have been several US-backed offensives on the remote island, but so far rebels leaders have not been captured.
A number of US troops are based in Jolo, where they are involved in counter-terrorism training.